A few years ago I asked the question: “Just how many unjust acts can a just war encompass before it ceases to be a just war?” This question assumed the connection between what scholars have defined as a distinction between ius ad bello and ius ad bellum, justness in the occasion for or cause of war and justness in the prosecution of war.
I just came across this definition of just war from the sixteenth-century Jesuit doctor Francisco Suarez:
In order that a war may be justly waged, certain conditions must be observed and these may be brought under three heads. First it must be waged by a legitimate power. Secondly its cause must be just and right. Thirdly just methods should be used, that is equity in the beginning of war, in the prosecution of it, and in victory (Tractatus de legibus, I, 9).
With Suarez’s definition in mind, I think we can summarize the matter thusly:
There is an important classical and scholastic distinction between ius ad bellum and ius ad bello, corresponding to the second and third heads of Suarez’s definition respectively. We might therefore say that a war can be just in a divided sense in two distinct ways: in its cause and in its execution. In this divided sense then Bainbridge is right to say that “violations of jus in bello do not affect the jus ad bellum question.”
But in the composite or compound sense of “a just war,” it must meet both conditions (as well as being pursued by a legitimate power, which is itself a complex question. Compare for instance Augustine’s question, “Set justice aside and what are kingdoms but large robber bands, and what are robber bands but little kingdoms?”).